It is known to manufacture such a preform by positioning a rod within a sleeving tube therefor to form an assembly in which there is an annular space between the rod and tube and collapsing the tube onto the rod to form a solid cross-section tube. Such a method is sometimes referred to as the `rod-in-tube` method and methods falling into this category are disclosed for example in GB2109367A and GB2043619A.
The collapsing step in the rod-in-tube method typically comprises sealingly closing the annular space between the rod and tube at one end of the assembly and applying suction to the annular space from the other end of the assembly. Typically the collapsing step also includes thermally softening the tube for example using a heat source which traverses the assembly from said one end to the other end and also the sealing of the annular space at said one end of the assembly has been achieved by a sealing device comprising elastomeric `O`-rings sealing against surfaces of the rod and tube at axially spaced locations. Such an arrangement is disadvantageous in that not only is the axial length of the assembly over which the sealing device extends not collapsed thus shortening the length of the preform, but also because when a heat source is used there is a danger that the material of the `O`-rings will degrade and that degradation products will be sucked along the annular space between the rod and tube during the collapsing step to contaminate the interface therebetween. Such contamination can result in flaws at the interface which give rise to breaks in the fibre drawn from the preform during tensile testing. In an attempt to avoid this latter problem the traverse of the heat source may be started at a predetermined distance from the `O`-rings, but this further shortens the resulting preform.